If you've got someone passing through Singapore they can get it there.
Only homeopathic melatonin is available in Australia but if you import it in quantities small enough for your own use, Customs are not interested. Say max 3 months supply.
I've found ..bodybuilding .com to be reliable suppliers of many supplements over a few years. I haven't ordered melatonin from them. I am in no way connected with them.
I declared a bottle of melatonin at Perth airport and the customs bloke just said "yeah?, is that it? go on through thanks sir."
